Drilling a natural gas or oil well involves the use of fluid (called drilling mud) or compressed air (air drilling) to aid in the process of drilling a borehole into the earth. Drilling mud is necessary in drilling because it cools and lubricates the drill bit, helps stabilize the well bore during drilling and keeps fluids in the formation from entering the borehole.
Drilling mud also is used to carry rock and soil (called drill cuttings) excavated by the drill bit up to the surface. At the surface, the drilling mud is separated from the drill cuttings. Drilling mud is valuable to the drill operator and is recovered so that it can be used again. Drill cuttings are produced as the rock is broken by the drill bit advancing through the rock or soil; the cuttings are usually carried to the surface by drilling fluid circulating up from the drill bit. Drill cuttings can be separated from liquid drilling fluid by shale shakers, by centrifuges, or by cyclone separators, the latter also being effective for air drilling. In cable-tool drilling, the drill cuttings are periodically bailed out of the bottom of the hole. In auger drilling, cuttings are carried to the surface on the auger flights.
The cuttings separated from the mud at the shale shakers may be coated with so much mud that they are unsuitable for the next reuse or disposal step or are difficult to handle or transport. Constituents of the cuttings or the mud coating them (e.g., oil, metals) may leach from the waste, making them unsuitable for land application or burial as a means for disposal. Methods for placing these constituent materials (contaminants) beyond the reach of a percolating liquid that might leach the materials into the surrounding soil or into the water table include solidification and stabilization.
Because of environmental concerns, the North Dakota Health and Energy Commission no longer allows open reserve pits at or near well sites once the sites are no longer used in the North Dakota oil and gas industry. The need to process, transport and dispose of drill cuttings however, remain an important and expensive aspect of the extraction of oil and gas.
Solidification involves the processes that encapsulate contaminated material to form a solid material and restricts contaminant migration by decreasing the surface area exposed to leaching and/or by coating the contaminated material with low permeability materials. Solidification is accomplished by mechanical processes, which mix the contaminated material with one or more reagents. Solidification does not necessarily involve a chemical interaction between the wastes and the solidifying reagents but may mechanically bind the waste into the monolith. Contaminant migration is restricted by vastly decreasing the surface area exposed to leaching or by isolating the wastes within an impervious capsule. By current convention, solidification is the addition of absorbents such as soil, fly ash, kiln dust, cement, clays or sawdust to the drilling waste. The solidified product will have improved properties such as strength, compressibility and a reduced permeability.
“Stabilization” involves the processes where chemical reactions occur between the reagents and contaminated material to reduce the leachability of contaminated material into a stable insoluble form. Stabilization chemically binds free liquids and immobilizes contaminated materials or reduces their solubility through a chemical reaction. The physical nature of the contaminated material may or may not be changed significantly by this process. Stabilization refers to those techniques that reduce the hazard potential of a waste by converting the contaminants into their least soluble, mobile, or toxic form. The physical nature and handling characteristics of the waste are not necessarily changed by stabilization. It is an adsorption process whereby contaminants are electrochemically bonded to the stabilizing agents. Contaminants that are stabilized within the matrix are less likely to be released into the environment. This process is particularly relevant to the stabilization of heavy metals and oils that would otherwise be rapidly available to the environment through leaching.
To isolate the reactive portions of drill cuttings from the environment, there is, then, an ongoing need to solidify on-site reserve pit fluids (drill cuttings, mud and other contaminants that are naturally occurring or part of the drilling process) that are generated directly from exploration (drilling). During exploration, a Drilling Mud Engineer will regularly dispose of drilling mud materials which have been down hole when, in the engineer's judgment such materials are too contaminated to continue to be useful. At such a point, these materials are placed in a reserve pit awaiting further processing for disposal.
Disposal wells represent a second significant point in the waste stream process for which a solidification need exists. The waste received at these disposal wells vary in its liquid content, with some waste being completely liquid, and some mixed with other constituents, including oil, mud and other material. The current process includes separating the latent oil that remains in the production water before any other processing occurs. This oil is then sold into the larger market. The remaining liquid requires further processing. It is currently common industry practice to mix this waste water with one of several cementitious products, to stabilize that waste, rendering it inert in its environment. Once inert, the engineer injects drilling wastes into underground formations for permanent disposal. Slurry injection technology involves grinding or processing solids into small particles, then mixing them with water or some other liquid to make a slurry. The slurry is then injected into an underground formation at pressures high enough to fracture the rock.
Currently, the predominant product used to stabilize the waste is fly ash. Fly ash is a coal combustion by-product, and comprises fine particles that are driven out of the boiler with the flue gases. Ash that falls in the bottom of the boiler is called bottom ash. Fly ash is generally captured by electrostatic precipitators or other particle filtration equipment before the flue gases reach the chimneys of coal-fired power plants. Depending upon the source and makeup of the coal being burned, the components of fly ash vary considerably, but all fly ash includes substantial amounts of silicon dioxide (SiO2) (both amorphous and crystalline), aluminum oxide (Al2O3) and calcium oxide (CaO), the main mineral compounds in coal-bearing rock strata. Proportions of these compounds in the fly ash depend upon the specific coal bed makeup, but may include one or more of the following elements or substances found in trace concentrations (up to hundreds ppm): arsenic, beryllium, boron, cadmium, chromium, hexavalent chromium, cobalt, lead, manganese, mercury, molybdenum, selenium, strontium, thallium, and vanadium, along with very small concentrations of dioxins and PAH compounds. As stated above, fly ash, though a by-product of coal combustion and readily available where coal is burned, is extremely expensive to transport.
Historically, fly ash has been used most frequently as a solidification/stabilization additive for treating drill cuttings and other types of wet solids. Drill cuttings composed of drilling fluids (fresh water and oil-based liquid), dirt, clay and rock are created as the hole is drilled deeper and deeper. These materials are solidified two ways. One, as they come off the rig, they are mixed with fly ash and disposed of in the solids pit on the pad site. Second, the solid and liquid cuttings are hauled off-site in closed top containers. Once the material reaches a landfill, it is solidified with fly ash. Fly ash is the leading drying and solidification agent in, for example, the Bakken oil fields. It is widely used as it is capable of solidifying into cement and has hygroscopic (ability to absorb water) properties.
Fly ash is produced in boilers burning pulverized coal and is removed from the boiler exhaust gases by electrostatic precipitators, baghouses, or scrubber systems. Fly ash is a heterogeneous mixture of silicon, iron, and aluminum oxides, carbon, and calcium oxide. Because it is collected by separating the solids from the gasses that make up the exhaust; fly ash particles are typically smaller than 200 μm in diameter and the mean particle diameter for fly ash is often less than 50 μm.
U.S. Pat. Nos. 4,274,880 and 4,226,630 disclose means for disposing of hazardous liquid wastes in combination with fly ash. These both exploit the pozzolanic reaction which occurs when fly ash is introduced into a waste water slurry. In addition, neither of these patents disclose or suggest that such a cementitious mixture may be combined to dispose of landfill leachate. Specifically, U.S. Pat. No. 4,226,630 is directed towards the disposal of water-borne heavy metal sludge produced in metal processing and refining plants, by combining the sludge with a very specific type of fly ash formed through the combustion of sub-bituminous coal which is only mined in the “Powder River Basin” of Gillette, Wyo. U.S. Pat. No. 4,432,800 is directed towards the treatment of kiln dust used in pozzolanic reactions with calcium oxide, calcium hydroxide or sodium hydroxide. The treated kiln dust is used with a pozzolan such as fly ash and a filler to produce a durable mass. There is no disclosure or suggestion in U.S. Pat. No. 4,432,800 for using such a mixture for the stabilization of drill cuttings.
Three critical factors drive supply and demand in the fly ash market and are, therefore, determinative of price. The first of these is the trend marked by the generally declining quality of fly ash. Much of the decline can legitimately be blamed on government regulation requiring NOx control. To reduce the presence of NOx, lower temperature burners are used which, because of their flame geometry and the lower temperature, pass much more unburned carbon to the resulting fly ash. The presence of more unburned carbon in fly ash can significantly diminish the beneficial application as either of a solidifying additive or in concrete mixtures.
The second factor is value. The industry is beginning to realize the substantial value that fly ash has as a cement additive. Because fly ash improves the workability and pumpability of concrete, increases its density, increases its ultimate strength and improves its resistance to chlorine and sulfate attack more and more of the available fly ash is consumed by the concrete industry. Chemically, fly ash reacts with excess portlandite in Portland cement making it unavailable to react with sulfate or carbon dioxide thereby greatly shrinking the size of pores present in the concrete. It also displaces its equivalent weight in cement costing $70-$80 per ton. The price of high quality fly ash pozzolan is beginning to rise in areas where there are shortages and prices of $25-$30 per ton are not uncommon.
The third and most important market factor has to do with the inherent immobility of the resource. Fly ash is consumed and marketed regionally. Because fly ash is a bulky, heavy material, transportation to needy markets is economically unfeasible. For example, it costs between $0.10 and $0.13/ton/mile to transport fly ash by pneumatically loaded trailer truck. Thus, there is a limit as to how far it can be economical at such prices. While fly ash can travel less expensively by either rail or barge, loadout facilities and rail car availability can provide serious complications. When the variations of the building industry's activities and subsequent demand are added in, it is not at all difficult to understand how regional shortages of quality fly ash occur.
To substitute for fly ash at sites distant from coal combustion, various wellhead engineers have used cement, wood chips left from lumber production, or quick lime. Because each of these products has distinct uses, they have not been economically attractive as a solidifying agent. Where available, the most frequently used product is quick lime.
Calcium oxide (CaO), commonly known as quicklime or burnt lime, is a widely used chemical compound. It is a white, caustic, alkaline, crystalline solid at room temperature. The broadly used term “lime” connotes calcium-containing inorganic materials, in which carbonates, oxides and hydroxides of calcium, silicon, magnesium, aluminum, and iron predominate. By contrast, “quicklime” specifically applies to the single chemical compound calcium oxide. Calcium oxide which survives processing without reacting in building products such as cement is called free lime. Quicklime is relatively inexpensive. Both it and a chemical derivative (calcium hydroxide, of which quicklime is the base anhydride) are, however, important commodity chemicals. It has the facility for the drying of soils but, by itself, is not suitable for stabilization.
Even in the petroleum industry itself, quicklime has a role: water detection pastes contain a mix of calcium oxide and phenolphthalein. Should this paste come into contact with water in a fuel storage tank, the CaO reacts with the water to form calcium hydroxide. Calcium hydroxide has a high enough pH to turn the phenolphthalein a vivid purplish-pink color, thus indicating the presence of water in the tank.
According to the American Petroleum Institute (API), it is estimated that approximately 1.21 barrels of total drilling wastes are generated for every foot drilled at a well. From reviewing actual drilling logs from a standard horizontal well, that exemplary well being drilled to at a minimum depth of 11,000 feet down-hole, there are between 6 to 8 truck-loads of drying agents currently required. A truck load is equal to 22 tons of material (the term “Super Sack™” is used interchangeably with the measure “ton” because of the extremely common use of Super Sacks™ to transport such materials in one-ton increments). Thus, a single 11,000 ft. well would therefore require approximately 154 Super Sacks™ of drying ingredient.
According to the recent production data available from the Department of Mineral Resources (January 2016), there are currently approximately 1,200 wells waiting for completion, plus 12,818 wells currently producing in North Dakota. In April 2016, a large oil field trans-load facility in Williston, N. Dak., Red River Supply, transported 20 million pounds of cement and lime each week during Q1, 2016. In rough approximation, half of that amount was used for down hole cementing. Even as expressed in Super Sacks™, the demand is stunning.
What is needed in the art is a method and composition to provide a lower cost alternative to fly ash as a solidifying agent.